In mobile communication systems, such as systems utilizing Orthogonal Frequency Division Multiplexing (OFDM), Frequency Division Multiple Access (FDMA), Multi-Carrier Code Division Multiple Access (MC-CDMA) techniques or the like, reference signals are commonly used for channel estimation. The basic idea for this is that a reference signal known by both a transmitter and a receiver is transmitted from the transmitter to the receiver and then the receiver may compare the received reference signal with the known reference signal, and from this comparison determine the channel characteristics.
Hereafter the background of the disclosure and also the disclosed embodiments itself will, for illustrational purposes, be exemplified in terms of a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) cellular system using OFDM. Embodiments as disclosed herein may also applicable to other communication systems and transmission methods. The disclosed embodiments may be applicable to essentially any multi-carrier system. Terms like User Equipment (UE) or Node B in the following text should therefore be interpreted broadly to also include base stations, or any other node in a cell communicating with UEs or the like, and mobile stations, or any other type of mobile unit communicating with a Node B or the like.
In an OFDM system, two-dimensional reference signals are used for channel estimation, the two dimensions relating to time and frequency. A two-dimensional time-frequency space is determined, which defines the positions of the reference symbols. This two-dimensional time-frequency space is in 3GPP LTE given by Ericsson, “Downlink reference-signals”, 3GPP RAN1 Tdoc R1-063008, Seoul, Korea, October, 2006. (“Ericsson”) as:
Reference-Symbol Position (Sub-Carrier) within Sub-Frame #n
First reference symbols: Sub-carrier x(n)+k*6 (in OFDM symbol #0).
Second reference symbols: x(n)+k*6+3 (in OFDM symbol #4 or #5).
Alternatives
1. x(n) constant and cell common→No hopping/shift.
2. x(n) constant but cell specific→Cell-specific frequency shift.
3. x(n) time varying and cell specific→Cell-specific frequency hopping.
The reference symbols are modulated by two-dimensional reference symbol sequences, which we shall denote as Reference Symbol Sequence (RSS). In order to improve inter-cell interference randomization, different RSSs are used in different cells. To be able to utilize the reference symbols for channel estimation, the RSS first has to be identified by a UE in the system. This is done during a so called cell search procedure, which includes detection of a cell ID. The cell search typically consists of three steps: the first step performs synchronization using the synchronization channel, the second step detects a group ID and frame synchronization from the synchronization channel, and the third step detects the complete cell ID from the reference signal. According to Ericsson, there is a one-to-one relation between the two-dimensional reference-symbol sequence, RSS, and the cell ID.
In 3GPP LTE, the RSS is constructed as a product of a pseudo-random sequence (PRS) and an orthogonal sequence (OS). Some embodiments disclosed herein may focus on the construction of orthogonal sequences, but it may here be mentioned that a PRS design could in turn be constructed as a product of different sequences, as described in Motorola, “Simulation results for GCL based DL reference signals”, 3GPP RAN1 Tdoc R1-063055, Riga, Latvia, November, 2006 (“Motorola”). Typically, the same PRS would be used for all cells connected to and under control of the same Node B, whereas different OS may be used in the different cells controlled by that Node B. It is reasonable to assume that the PRS and the sequence x(n) should be determined from a so called group ID, obtained in a 2nd step of the cell search, as described in Nokia et al., “Outcome of cell search drafting session”, 3GPP RAN1 Tdoc R1-062990, Seoul, Korea, October, 2006 (“Nokia”), so that it can be used to re-modulate the reference signals (i.e., be cancelled). Thus, in that case the number of group IDs must be equal to the number of PRSs. The correct phase of a PRS is also known after the 2nd step, from the acquired frame synchronization. Thereby, after the 2nd step, the PRS is known and can be cancelled from the reference signal. Then the detected sequences in the 3rd step are orthogonal since only the OS remains in the reference symbols. The OS used can then be detected by correlating the values of received reference symbols with all possible OSs, in a 3rd step of the cell search.
However, there are a number of problems related to conventional reference sequences, and in particular to the orthogonal sequences (OSs). The conventional cell search procedure is limited by fixed rules regarding the characteristics of the OSs and the PRSs. These fixed rules make the design of the cell search procedure very inflexible, which may result in an inefficient cell search. There is thus a need for a cell search having a more optimized performance than the one shown in the related art.
There is further a radio transmission overhead problem in conventional solutions. It is, as always in radio communication systems, desirable to utilize the radio resources in the system as efficiently as possible and to mitigate the signaling overhead in the system.
Further, if a Node B controls many cells, OSs may have to be reused in conventional systems. This may result in problems related to OS reuse, such as OS reuse planning and high levels of pilot-to-pilot interference between neighboring cells or sectors having reused OSs.